Both external (i.e., exposed) and internal (i.e., enclosed) drivetrains are common in the modern field of bicycle drivetrains. External drivetrains typically employ parallelogram-type derailleurs, a chain, and sprockets. Parallelogram-type derailleurs swing through various ranges of motion and are therefore difficult to completely enclose. The rear parallelogram-type derailleur bolts to the outside of the bicycle frame near the rear wheel hub. From this location, the parallelogram of the rear derailleur can swing even further outboard.
External drivetrains have the advantage that the number and overall range of drive ratios is not limited by the requirement that the mechanisms fit inside any wheel hub or enclosure. Typically, external drivetrains employ two or three chain rings (front sprockets) cooperating with a front derailleur. The front shifting system gives the major ranges of gearing, and a rear derailleur cooperating with rear sprockets provides the fine tuning with smaller increments of drive-ratio shifting. The rear derailleur typically controls the chain destination among seven or eight rear sprockets associated with a rear wheel. The overall drive ratio variation typically approaches 4:1.
Internal drivetrains usually take the form of an internally geared hub. The hub may be mounted near the bottom bracket using a jackshaft but is usually integral with the hub of the rear wheel. Internal gear hubs typically use planetary gearing and normally provide from three to nine speeds. Internal gear hubs have been produced with as many as twelve or fourteen speeds, but the additional weight associated with the rear wheel makes these internal gear hubs less than ideal.
The weight of a rear hub with internal gearing is acceptable for pavement riding, but undesirable for off-road riding. This has given rise to attempts to relocate it to a jackshaft near the bottom bracket, closer to the center of gravity of the bicycle. Unfortunately, the planetary gearing and jackshaft bearings reduce the efficiency of the system compared to a standard external drivetrain.
There are many tradeoffs between external and internal drivetrains. External has good gearing range and good efficiency, but is exposed to wet and mud, which can drastically reduce the efficiency and increase wear on the drivetrain. Also, the exposed drive train is vulnerable to derangement due to impact or even a small object such as a twig getting stuck in the chain, which can easily obstruct the idler pulleys, and cause the rear derailleur to break off.
Improvements to external drivetrains, such as indexed shifting and special features in chains and sprockets to promote clean and prompt shifting, have made the external drive train shiftable under load in most circumstances. The only condition under which a typical modern external drivetrain shifts poorly is when the front derailleur is shifted under load. The problem is inherent that the front derailleur is set up to shift the loaded side of the chain, in contrast to the rear derailleur, which manages the slack side of the chain, making rear shifting generally much easier.
Internal gear hubs are of course enclosed, and the mechanisms live in an oil or grease bath substantially free of contamination.
Internal gear hubs change ratios without the need to alter the chain path between multiple sprockets. The chain can easily be enclosed in a reasonably slim chain guard. Therefore, the shifting mechanism, sprockets, and chain are completely protected from the elements and from contaminants. This makes these bicycles very suitable for commuting in all conditions and storing outdoors.
Current Technology of Front Shifting
Bicycle drivetrains include both external and internal drivetrains. External drivetrains typically use derailleurs to transfer a drive chain from one sprocket to another to achieve various drive ratios. Internal drive trains use planetary gearing commonly situated inside the hub of the rear wheel. In recent years, planetary gears have also been located in the space of the front sprockets.
The most common form of bicycle gearing is external gearing associated with the rear hub. When it is desired to further increase the available drive ratios, external gearing associated with the front pedal crank can be added. This usually takes the form of two or three sprockets bolted to the drive side pedal crank. A front derailleur is secured to the frame. The drive chain passes through the front derailleur cage. The front derailleur places the cage in various positions to cause the drive chain to transfer to the desired chain ring to effect a shift. The front derailleur is in turn controlled by a control cable which is actuated from the handlebar. The actuator is called a “shifter” and usually takes the form of a lever or twist grip system.
The arrangement of three sprockets (also known as “chain rings”) bolted to the drive side pedal crank is called a “triple chain ring.” The overall drive ratio variation provided by the common “triple chain ring” is about 2:1.
Problems with the Triple Chain Ring System
Several problems are associated with the triple chain ring system. For example, it is difficult to enclose and is degraded by sand and mud when used off-road.
Another problem is that the triple chain ring system requires the front derailleur to forcibly move the taut, load-bearing part of the chain. Thus, shifting the front derailleur requires more effort and is less reliable than shifting the rear derailleur, which manages the slack part of the chain. Riders must plan ahead and shift when not applying a full load. When riders attempt to shift under load, the chain may fail to shift or derail completely off the chain rings.
The lateral spacing of the three chain rings creates oblique chain lines when riding “crossed over.” This overstresses chains and causes excessive wear and friction.
Innovative bicycle designers often have to arbitrarily place a stubby vestigial seat tube on the bottom bracket to support the front derailleur. This is the case even when the frame design supports the seat with no seat tube.
Still another problem is that the front derailleur must be adjusted vertically and angularly on the seat tube in relation to the chain rings. The limit screws limiting the range of motion of the front derailleur cage must also be adjusted. Mechanics commonly use pliers to forcibly reconfigure the chain cage to a shape in which they believe it will work better.
The triple chain ring system has been evolving and in common use since the early days of mountain biking. Although it is relatively cheap and light, the triple chain ring simply does not work well under pedaling loads. Industry experts have complained of this problem for years. Unreliable front shifting is one of the industry's biggest embarrassments. This has been a barrier to expanding the bicycle market to a wider audience.
A need therefore exists for a reliable bicycle drivetrain that is suitable for harsh outdoor riding and storage conditions, while providing a sufficient range of usable gearing for any application, including but not limited to road biking and mountain biking.